1. Field of the Invention
The present invention relates to a photopolymerization initiator. More particularly, the invention relates to a photopolymerization initiator to be used for a photopolymerizable resin composition which is cured by photopolymerization in only a part exposed to an activating ray and which has an excellent adhesion to a base material.
2. Prior Art
Photopolymerizable resin compositions have been used as starting materials for inks of activating ray curing type, paints and photoresists. They have a serious defect that the curability of coating films of them are poor upon exposure to the light. Namely, the thickness of the coating film exerts a great influence on the curability. For example, the coating film of photoresist or screen printing ink is as thick as 10 to 100 .mu.m and, therefore, the inner surface of the coating film is not cured sufficiently. This phenomenon is remarkable when a pigment or a dye is incorporated in the photopolymerizable resin composition and a tack remains on the coating film surface. Thus, the expected curability cannot be obtained.
Processes have been proposed for solving the problem of curability by using a combination of photopolymerization initiators such as a combination of benzophenone or its derivative with 4,4'-bis(diethylamino)benzophenone (Japanese Patent Publication Nos. 37902/1978 and 25943/1979) and a combination of xanthone with a 4,4'-bis(dialkylamino)benzophenone (Japanese Patent Laid-Open No. 92246/1977).
Though these photopolymerization initiators are effective, their effects are yet insufficient particularly when a dye or a pigment is used, since with a coating film thickness of more than 10 .mu.m, the curability becomes insufficient and the adhesion with a base material is reduced.